It's more accurate to say that every single Praxis kit in the game is missable, except the two in the LIMB clinic in Panchaea. As soon as you leave a city hub, you'll lose Praxis kits from its sidequests; as soon as you leave a city hub for the second time, you'll lose Praxis kits that are just out there to find.
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jpreteSep 5 '11 at 17:55

For example the first mission (to the Sarif factory) contains one, as do your visits to Tai Yong Medical and Picus (and possibly more "mission" locations), all these permanently lost when you leave the respective levels if you haven't picked them up.

A few side missions in the city hubs also award Praxis kits. Given that side missions expire permanently when you leave the hub, these kits can be lost if the side mission is not completed before leaving.

And, obviously, those kits found hidden around the city hubs are lost if you haven't collected them when you finally leave the hub for the final time.

This includes kits that may be purchased from LIMB clinics. If you don't buy them, they're gone forever once you leave the hub - even if you return to the same hub later the clinic will only have 2 new kits, the old ones are lost.